MongoDB query: keys and their distinct values (each key independently)?

If you have this collection of objects:

{ "a": 10, "b": 20, "c": 30 }
{ "a": 11, "b": 20, "c": 31 }
{ "a": 10, "b": 20, "c": 31 }

1. There is a way to get distinct values, for example, for field "a":

   [10, 11]
   

2. There is also a way to get distinct values of any tuple, for example, for pairs of ("b", "c"):

   [
       {"b": 20, "c": 30},
       {"b": 20, "c": 31}
   ]
   

Is there a way to query distinct values for each field individually in a single query?

For example, I can simply use query 1 above 3 times for "a", "b", "c":

[10, 11]
[20]
[30, 31]

But I guess it might be less efficient and there should be a better option.

Bonus: How to do it if the list of fields is not known upfront?

Ideally, the single query should return all keys and their distinct values:

{
    "a": [10, 11],
    "b": [20],
    "c": [30, 31]
}

Assuming you don't know the full list of the fields beforehand, you need to use $objectToArray to convert the $$ROOT document into an array of k-v tuples. Then group by the field name and $addToSet the values.

db.collection.aggregate([
  {
    "$project": {
      _id: 0,
      arr: {
        "$objectToArray": "$$ROOT"
      }
    }
  },
  {
    "$unwind": "$arr"
  },
  {
    $match: {
      "arr.k": {
        $ne: "_id"
      }
    }
  },
  {
    $group: {
      _id: "$arr.k",
      values: {
        "$addToSet": "$arr.v"
      }
    }
  }
])

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